K. G. Evans

Medium-term erosion simulation of an abandoned mine site using the SIBERIA landscape evolution model

This study forms part of a collaborative project designed to validate the long-term erosion predictions of the SIBERIA landform evolution model on rehabilitated mine sites. The SIBERIA catchment evolution model can simulate the evolution of landforms resulting from runoff and erosion over many years. SIBERIA needs to be calibrated before evaluating whether it correctly models the observed evolution of rehabilitated mine landforms. A field study to collect data to calibrate SIBERIA was conducted at the abandoned Scinto 6 uranium mine located in the Kakadu Region, Northern Territory, Australia. The data were used to fit parameter values to a sediment loss model and a rainfall–runoff model. The derived runoff and erosion model parameter values were used in SIBERIA to simulate 50 years of erosion by concentrated flow on the batters of the abandoned site. The SIBERIA runs correctly simulated the geomorphic development of the gullies on the man-made batters of the waste rock dump. The observed gully position, depth, volume, and morphology on the waste rock dump were quantitatively compared with the SIBERIA simulations. The close similarities between the observed and simulated gully features indicate that SIBERIA can accurately predict the rate of gully development on a man-made post-mining landscape over periods of up to 50 years. SIBERIA is an appropriate model for assessment of erosional stability of rehabilitated mine sites over time spans of around 50 years.

Methods for assessing mine site rehabilitation design for erosion impact.

Erosion of rehabilitated mines may result in landform instability, which in turn may result in exposure of encapsulated contaminants, elevated sediment delivery at catchment outlets, and subsequent degradation of downstream water quality. Rehabilitation design can be assessed using erosion and hydrology models calibrated to mine site conditions. Incision rates in containment structures can be quantified using 3-dimensional landform evolution simulation techniques. Sediment delivery at catchment outlets for various landform amelioration techniques can be predicted using process-based and empirical erosion-prediction models and sediment delivery ratios. The predicted sediment delivery can be used to estimate an average annual stream sediment load that can, in turn, be used to assess water quality impacts. Application of these techniques is demonstrated through a case study applied to a proposed rehabilitation design option for the Energy Resources of Australia Ltd (ERA) Ranger Mine in the Northern Territory of Australia.

Post‐mining landform evolution modelling: 1. Derivation of sediment transport model and rainfall–runoff model parameters

Data were collected from three sites on the waste rock dump at ERA Ranger Mine: (1) the cap site which was unvegetated and unripped with a surface slope of 2·8 per cent; (2) the batter site, surface slope 20·7 per cent, also unvegetated and unripped but with a covering of coarse rock material; and (3) the soil site, surface slope 1·2 per cent, which had c. 90 per cent vegetation cover of low shrubs and grasses and had been topsoiled and surface ripped. Natural rainfall events were monitored on the sites to collect rainfall, runoff and soil loss data to parameterize a sediment transport model of the form T = β S ∫ Q dt, and the DISTFW rainfall–runoff model. Low frequency, high intensity events resulted in the greatest soil loss. To accurately predict sediment loss during high intensity events, storms with a range of intensities were selected to derive the sediment transport model. DISTFW hydrology model parameters were derived by fitting four monitored events simultaneously. The selected parameters for the vegetated and ripped case may overpredict discharge for some rainfall events resulting in conservative design of erosion control features on rehabilitated landforms. Copyright 2000

Post‐mining landform evolution modelling: 2. Effects of vegetation and surface ripping

Computer simulations of the topographic evolution of the proposed post-mining rehabilitated landform for the ERA Ranger Mine, showed that for the unvegetated and unripped case, the landform at 1000 years would be dissected by localized erosion valleys (maximum depth = 7·6 m) with fans (maximum depth = 14·8 m) at the outlet of the valleys. Valley form simulated by SIBERIA has been recognized in nature. This indicates that SIBERIA models natural processes efficiently. For the vegetated and ripped case, reduced valley development (maximum 1000 year depth = 2·4m) and deposition (maximum 1000 year depth = 4·8m) occurred in similar locations as for the unvegetated and unripped case (i.e. on steep batter slopes and in the central depression areas of the landform). For the vegetated and ripped condition, simulated maximum valley depth in the capping over the tailings containment structure was c. 2·2 m. By modelling valley incision, decisions can be made on the depth of tailings cover required to prevent tailings from being exposed to the environment within a certain time frame. A reduction in thickness of 1 m of capping material over tailings equates to c. 1 000 000 Mm3 over a 1 km2 tailings dam area. This represents a saving of c.

A preliminary typology of Australian tropical rivers and implications for fish community ecology

1 500 000 in earthworks alone. Incorporation of SIBERIA simulations in the design process may result in cost reduction while improving confidence in environmental protection mechanisms. Copyright 2000

Gully Initiation and Implications for Management of Scour Holes in the Vicinity of the Jabiluka Mine, Northern Territory, Australia

Australian tropical rivers are dependent on highly seasonal rainfall and runoff between November and April. They also transport low sediment and solute loads, except where catchments have been disturbed by mining, grazing and cropping. Aquatic habitats or channel units are the physical template influencing fish communities. Pools provide dry-season refuges for fish and channels provide pathways for movement between refuges when streamflows are re-established. A preliminary geomorphological typology of Australian tropical river reaches (excludes estuaries) is proposed that defines nine distinct river types: (1) bedrock rivers (upland channels and gorges); (2) bedrock-confined rivers; (3) avulsive rivers; (4) meandering rivers (confined meandering, laterally migrating unconfined and laterally stable unconfined); (5) straight rivers; (6) floodouts; (7) island- and ridge-anabranching rivers; (8) co-existent mud-braided and anabranching rivers; and (9) extensive freshwater wetlands and billabongs. Many of these have not been recognised overseas and are unique. Channel units differ greatly between river types and contribute to distinctive fish communities in different river types. As expected, fish diversity decreases upstream because of less diverse habitat and natural barriers to fish movement at steps, falls and turbulent cascades and rapids. Fish kills occur in most years and are caused by several different factors that reduce dissolved oxygen.

Using the RUSLE to identify factors controlling erosion rates of mine soils

Abstract A track across a burnt grass swale was used intensively on the Jabiluka Mineral Lease (located adjacent to Kakadu National Park in the seasonally wet tropics of the Northern Territory, Australia) for a short time period during the 1998 dry season. Repeated vehicle passes over the burnt grass increased soil bulk density and locally disrupted the root and algal mat, lowering the critical shear stress for sediment transport. Overland flow during the next wet season was above average and eroded eleven discontinuous, flow‐aligned scour holes in the wheel ruts where the track crossed grassed sandy swales. Although the site was burnt again during the next dry season, the scour holes did not coalesce during the second wet season, which was wetter than the previous one, because infrequent traffic bypassed the eroded section allowing grass to re‐establish. Scour holes on vehicle tracks in the Kakadu region are an intermediate but reversible stage in the development of gullies in grassed swales. Treatment of scour holes by soil conservation works may prevent gully formation.

Sediment Loss from a Waste Rock Dump, ERA Ranger Mine, Northern Australia

Observable differences in particle size, smoothness and compaction between cap site (slope 2·8 per cent) and batter site (slope 20·7 per cent) surfaces on the waste rock dump at Ranger Uranium Mine were quantified in terms of revised universal soil loss equation (RUSLE) parameter values. Cap site surface material had a Km (erodibility corrected for sediment density) of 0·030 and batter site surface material had a Km of 0·0056. Using these Km values (derived from particle size distributions), slope length and steepness (LS) factors of 0·36 for the cap site and 3·66 for the batter site, and a cover (C) factor of 0·45 for the cap site and 0·16 for the batter site, the RUSLE predicts an erosion rate from the cap site that is 1·9 times greater than erosion from the much steeper batter site. The RUSLE indicates that the finer particle size and blocky soil structure of the cap site (D50 = 0·91 mm) compared with the looser granular structure of the batter site (D50 = 1·74 mm) strongly influence erosion. The predictions are similar to observed soil losses from erosion plots on these sites under rainfall simulation events, for which the measured erosion rate from the cap site was approximately twice that from the batter site. For the RUSLE to predict the observed erosion rates, the support practice (P) factor for the cap site would have to be approximately 30 per cent greater than the P factor for the batter site. The higher cap site P factor probably results from smoothing and compaction caused by vehicle movement across the surface. Compaction is considered to have greatly reduced infiltration capacity, thus increasing the erodibility of the cap site. Vehicles probably also crushed the surface material at the cap site, creating the observed finer particle size distribution and further increasing the erodibility. Compaction, through its effects on erodibility (Km) and surface roughness (P), is concluded to be the major cause of higher erosion from the cap site, even though the slope steepness is 10 times less. Parameterisation of the RUSLE quantifies the differences between sites and explains the unexpected erosion rates observed. The results highlight the need for careful management of rehabilitated sites to avoid increases in erosion which may arise from compaction by machinery.

Suspended Sediment Concentration-turbidity Relationships for Ngarradj - a Seasonal Stream in the Wet-dry Tropics

During the 1994/95 wet season, runoff, suspended load and bedload loss from large scale erosion plots under natural rainfall events were measured at three sites with different treatments on the Energy Resources of Australia Ranger waste rock dump (cap, soil, fire). The fire site has well established trees, the soil site smaller shrubs and the cap site minimal vegetation. All three sites are located on the flat areas of the waste rock dump with similar slopes and cap, fire and soil are local names for the sites. The quantity of bedload eroded from the soil and fire sites decreased during the monitoring period, however, the same trend was not observed on the cap site. Bedload loss from the fire and soil sites is significantly log-linear with time and reduces at a decreasing rate until becoming statistically constant toward the later part of the first half of the wet season. For corresponding storms at each site, bedload erosion was highest from the unvegetated and unripped cap site and lowest from the well vegetated fire site. Site specific linear relationships between bedload and total sediment load have been derived and can be used to derive total sediment load in the absence of measured suspended load.

Sediment fluxes and sinks for Magela Creek, Northern Territory, Australia

Abstract In order to assess the impact of mining-related erosion on stream suspended sediment loads within a catchment, it is critical that suspended sediment movement is accurately measured. The technique of using turbidimeters to monitor stream suspended sediment concentration was assessed using data collected at three gauging stations within a small catchment in the Kakadu region. Turbidity data were strongly correlated with mud concentration data collected at all three sites. Futhermore, analysis of periods of high sediment movement associated with runoff events showed that the use of turbidimeters is considered essential for estimating individual event loads within the region.